Shield mastery, increased defences

it's base armour * (1 + all "% increased modifiers") * (1 + " % more modifier)" * (1 + " % more modifier) ....

all "increased" (and "reduced") modifiers to one stat stack additively. "more" (or "less") is used for those that stack multiplicatively. This is true for all stats in the game.

both "increased armour" (such as from passives) and "increased defences" are "increased" bonuses applying to your amrour value. Determination is a "more" bonus.
so the amount of armour from your shield is:
base shield armour * (1 + increased armour + increased shield defences) * (1 + determination)


Note that increased armour mods on the shield item itself are modifying the base armour of the item, not the armour you get from it, so are already applied before we get to this point, to actually get the "base armour" value.